Don't override zsh users' compinit options.

I call "compinit -i" in my .zshrc, to allow loading autocompletion from
"unsafe" locations. envsetup.sh breaks that. Change behavior to only
call compinit if bash-style completion is not already enabled.

This is a minimal fix; an alternative would be to rely on the user
to call 'bashcompinit' in their .zshrc instead, and just skip all
autocomplete functionality if they don't.

Test: manual - source ./envsetup.sh in {bash, zsh, zsh -f} and test
	completion for lunch
Change-Id: Ibb10eb68a96d69c19b9db9b21e0546fe8ae299b2